This invention relates to fuel storage tanks, especially gasoline/petrol storage tanks and, in particular, is concerned with means for reducing the generation of fuel vapour during filling of such tanks.
In conventional installations, a fuel pipe extends into the storage tank from outside and has an outlet situated close to the bottom of the tank. When the tank is refilled or topped up, e.g. from a road tanker, fuel is delivered at a high flow rate by gravity feed through the fill pipe. This displaces vapour contained in the tank which is either recovered via a vapour recovery line back to the tanker or is vented to atmosphere via a vent valve.
During the start of the delivery process the fuel delivered by the tanker displaces the air mass or slug in the fill hose and fill pipe into the tank below the liquid level. In these circumstances, a great deal of fluid turbulence is generated in the tank which causes a break up of the stratified high density vapour layers immediately above the liquid surface in the tank. As a result, high density vapour is mixed with low density ullage vapour, increasing the particle count or average vapour density level of the displaced tank vapour. As high density vapour contains a significant fuel volume and fuel volume is metered directly into the tank by the tanker delivery system, this constitutes a loss by the owner of the filing station.
A secondary effect of passing a volume of air through liquid fuel is the direct generation of fuel vapour, thus adding to the total vapour volume and subsequent losses. This effect, combined with the disturbance of the vapour layers previously discussed, increases the ullage vapour pressure causing the vapour vent relief valve to open and remain open, even when fuel filling is completed again adding to vapour (fuel) losses, an environmental and economic concern.
EP-0327518 describes one solution to this problem. In the filling system proposed in this patent, holes are formed in the fill pipe above the normal liquid level in the tank so that the interior of the fill pipe communicates with the head space or ullage. A second, smaller inner tube is inserted concentrically into the fill pipe and has holes which connect its interior with the space between the two tubes. Fuel is filled via the inner tube and air and vapour passes through the holes in the inner tube into the space between the two tubes. Some vapour and air passes through the holes in the fill pipe into the head space, while vapour remaining between the two tubes is collected via a vapour recovery system.
GB Specification No. 2301347 describes a similar system in that the fill pipe is also fitted with a second, perforated inner tube. In this system, however, the fuel is introduced into the space between the fill pipe and the inner tube and air carried in with the fuel is stated to pass into the inner tube through the perforations, and vapour is collected from the inner tube via an external vapour collection system.
Both of the above systems are relatively costly and involve the introduction of a substantial number of pipework connections and potential fuel and vapour leakage points.
It is an object of the present invention to provide cost effective, integral means for controlling the turbulence and vapour generation during filling of fuel storage tanks, while avoiding the problems of the prior art.